In some telecommunication systems it is customary to provide transceivers with frequency discriminators designed to recognize a certain tone or tone sequence used for ancillary operations such as switching and supervision. Generally, in such instances, a considerable number of tone frequencies (e.g. several tens) are utilized for various ancillary functions. It thus becomes necessary to employ a corresponding number of fixed narrow-band filters to recognize the respective tone frequencies or to use a single adjustable filter for this purpose.
The retuning of filters by the adjustment of an impedance in an L/C or R/C network requires high-precision components properly calibrated to pass the desired tone frequency with a given setting. Frequency discriminators of this type, though less complex than those including a multiplicity of fixed-band filters, are therefore relatively expensive.